CAN CHALCOPHILE TRACE ELEMENTS IN PYRITE ASSOCIATED WITH THE LADY LORETTA DEPOSIT DEFINE USEFUL PRIMARY DISPERSION HALOS?

The Lady Loretta deposit (13.6Mt at 17.1wt% Zn, 5.9wt% Pb and 97g/t Ag) occurs in pyritic shales and siltstones in northwest Queensland. The base metal sulfides and pyrite are thought to form through a combination of exhalative–syngenetic and diagenetic processes occurring during deposition of the host sequence. Pyrite is ubiquitous in the ores, and forms an extensive primary dispersion halo around the orebody.

At Lady Loretta low sulfide siltstones and shales display broad chemical dispersion halos (notably, for Tl, Mn, and Fe) that can be identified for tens of metres vertically and hundreds of metres laterally around the deposit. Hence, mineralising processes operated for some time. In contrast, to the siltstones and shales, conventional analyses of pyrite bands do not reveal anomalous levels of chalcophile elements in samples collected over about 180 metres vertical thickness around the orebody.

One explanation for the apparent lack of a halo may lie in the difficulty of defining true ‘background’ levels for chalcophile elements in pyrite when using bulk samples. Alternatively, the interplay between hydrogenous and hydrothermal processes may produce a complex chalcophile element signature in diagenetic pyrite.

Here we present new quantitative chalcophile trace element data for Lady Loretta pyrite samples obtained using a novel Laser Ablation-ICPMS technique. This technique permits sampling of areas as small as 30 microns diameter, and largely avoids the heterogeneity problems of the previous bulk samples. Better estimates of background values will be presented for several elements, and these may help discriminate subtle changes in pyrite chemistry associated with the onset of the base metal mineralising system.